1. Field of the Invention
The present invention relates to an exposure apparatus for image formation that is applied to an image forming apparatus such as color copying machine and printer for forming an image by means of electrophotography.
2. Description of the Background Art
An image forming apparatus forming an image through electrophotography converts a latent electrostatic image formed on the surface of a photoreceptor in an exposure process into a visible developer image in a subsequent development process. With recent progress in the enhancement of resolution, strong requests have been made to this type of image forming apparatus for a higher image quality as well as a higher reproducibility. Accordingly, various studies and developments as well as a great number of technological innovations have been made regarding the image formation technique over a wide range, for example, retrieval and quantification of parameters contributing to enhancement of the image quality, improvement of image processing technique, improvements of energy modulation and control technique, and the like.
Regarding a technique of forming a latent electrostatic image on the surface of a photoreceptor, an improvement is accomplished by optimizing the spot diameter of an exposure spot on the photoreceptor surface with respect to a dot pitch determined by resolution, and image formation is thus achieved in an improved state.
For example, Japanese Patent No. 2,668,440 discloses a technique employed in an image forming apparatus that uses pulse-width modulation, according to which a halftone image is formed by full-image exposure of pixels and, in order to prevent a drastic change in density due to cross talk of adjacent spots, the maximum spot diameter per pixel is made smaller than the size 0.7 times as large as one pixel size. In this way, halftone is accurately reproduced.
According to a technique disclosed in Japanese Patent Laying-Open No. 7-61036, the spot diameter of an exposure spot is set in a range from 1.0 to 1.3 times a dot pitch and the maximum exposure amount of the exposure spot is set to at least 11 times a luminous sensitivity coefficient on the surface of a photoreceptor. A black image of a large area having a high density and evenness can thus be generated and exposed isolated dots and non-exposed isolated dots are suitably reproduced.
According to a technique disclosed in Japanese Patent Laying-Open No. 58-152269, all pixels are exposed with the spot diameter of an exposure spot that is set to 1.3 to 1.9 times the distance between scan lines. The difference in exposure energy density between the area on a scan line and the area between scan lines is thus decreased to lessen nonuniformity of exposure. A properly reproduced image is accordingly achieved.
According to a technique of forming a latent electrostatic image on the surface of a photoreceptor, the image consisting of a collection of dots, the exposure energy is modulated according to the number of adjacent dots to enable enhancement of image reproducibility with respect to various image data and accordingly a high-quality image can be produced regardless of the type of image data.
For example, according to a technique disclosed in Japanese Patent Laying-Open No. 9-85982 employing a photoreceptor with a high xcex3-value, an exposure amount for a dot of interest is decreased as the number of dots adjacent horizontally and vertically to the dot of interest increases.
In addition, according to a technique disclosed in Japanese Patent No. 2,839,006 regarding control of binary image data, a pulse width of image data is increased with respect to character data and isolated-dot data in a pseudo halftone region, and the pulse width is decreased with respect to continuous-dot data in the pseudo halftone region so as to achieve clear and smooth characters as well as an excellent tone reproduction.
In order to simultaneously accomplish image reproducibility of an isolated dot and an isolated line in view of enhancement of image quality, the technique disclosed in Japanese Patent No. 2,668,440 could be used to achieve an adequate image reproducibility by performing exposure with an exposure spot diameter smaller than a pixel size without modulating energy. However, as the image density increases, a lens of an expensive material that is precisely processed is required. Further, the lens must be positioned precisely because the depth of focus of an exposure spot is smaller. Resultant problems are a considerable difficulty of the manufacturing technique and an increase of cost caused by a decreased yield.
The techniques disclosed in Japanese Patent Laying-Open No. 7-61036 and Japanese Patent Laying-Open No. 58-152269 could produce an image excellent in reproducibility by means of optimum dot-energy control according to image pattern. However, when the resolution exceeds 1200 dpi, the exposure spot diameter is less than 40 xcexcm. Then, in order to achieve such an exposure spot diameter, a lens must be processed and positioned with a high precision, resulting in increase of cost due to decrease in yield as the problem of the technique disclosed in Japanese Patent No. 2,668,440.
Additionally, the technique disclosed in Japanese Patent Laying-Open No. 9-85982 could adjust the dot diameter with a slight change in light quantity by employing a photoreceptor having a high xcex3-value. However, in formation of minute dots that is necessary for forming a high-resolution image, this technique is likely to be affected by variation of the dot diameter caused by change in the light quantity, resulting in a problem of difficulty in control of the light quantity.
Further, an image forming apparatus of the electrophotography system generally uses a laser beam with an exposure spot diameter of 60 to 80 xcexcm in an exposure process for forming a latent electrostatic image. This size is three to four times the dot pitch (about 20 xcexcm) for a resolution of 1200 dpi. Then, cross talk between peripheral dots becomes conspicuous resulting in deterioration in image quality.
For example, regarding a periodic line pattern consisting of paired image lines (black lines) and non-image lines (white lines) per pixel, the non-image lines between image lines intended to be formed are not reproduced due to interference of adjacent lines with respect to each other, and a resultant image is solid black as a whole. If the exposure energy of the laser beam is reduced for avoiding this, latent image potential does not attain exposure energy density potential and thus no image is formed.
Similarly, regarding image data having an isolated-dot pattern where no dot is present around a dot of interest and an isolated-line pattern where a plurality of dots continue in one direction, there could occur situations in which the isolated-line pattern can be formed while the isolated-dot pattern cannot be formed and in which the isolated-dot pattern can be formed while the isolated-line pattern has an increased line width. Accordingly, adjustment of exposure energy does not accomplish image reproducibility for both of the isolated-dot pattern and the isolated-line pattern at the same time.
In particular, techniques which have actually been employed frequently reach a result where the isolated-line pattern can be formed while the isolated-dot pattern cannot be produced. Then, in order to achieve an image of a high resolution exceeding 1200 dpi and a high quality by adjustment of exposure spot diameter, an optimum exposure energy should be defined for each pattern. Even if an exposure spot diameter can be obtained that is close to the dot pitch for the resolution of 1200 dpi or higher by reducing the exposure spot diameter, it would be difficult to achieve image reproducibility for both of the isolated-line pattern and the isolated-dot pattern by means of binary exposure energy control.
When a thick photosensitive layer is employed at the surface of a photoreceptor, image reproducibility conspicuously deteriorates. If the photoreceptor uses a thin photosensitive layer for enhancement of image reproducibility, deterioration of dark attenuation characteristics and photosensitive characteristics due to wear as well as increase of residual potential occur in an early stage, resulting in a shorter life. Further, the thin-layer photoreceptor has a relatively low and restricted charge potential on the surface of the photoreceptor, and the potential contrast of a latent electrostatic image deteriorates. Then, tone of sufficient solid-black density and image density cannot be achieved.
One object of the present invention is to provide an exposure apparatus for image formation capable of achieving image reproducibility with a highimage quality for both of an isolated-line pattern and an isolated-dot pattern of an image having a high resolution of at least 1200 dpi, without causing increase of cost due to requirements for high precision in lens processing and positioning and without shortening life of a photoreceptor due to reduction in the thickness of a photosensitive layer.
An exposure apparatus for image formation and an image formation method according to the invention for achieving the above object relate to an apparatus and a method that use an exposure spot to scan a surface of a photoreceptor to which a predetermined potential of a single polarity is uniformly applied so as to form a latent electrostatic image on the surface. The invention is characterized in that exposure energy of different values is applied respectively to an isolated-dot pattern and an isolated-line pattern of image data, when exposure spot diameter D and dot pitch P have a relation D greater than 1.4 P, the isolated-dot pattern including a dot surrounded by no dot while the isolated-line pattern including a line formed of a plurality of dots arranged continuously in one direction.
According to the invention, when exposure spot diameter D exceeds the value which is 1.4 times as large as dot pitch P, exposure energy of optimum values is applied respectively to the isolated-dot pattern and the isolated-line pattern. Then, without use of a lens processed and positioned precisely or a photoreceptor having its photosensitive layer decreased in its thickness for forming an image of a high resolution, a high-quality image can be reproduced with respect to both of the isolated-line and isolated-dot patterns. Accordingly, an image excellent in reproducibility can be formed without increase of cost and shortening of photoreceptor life.
According to the invention, when exposure spot diameter D and dot pitch P have a relation D greater than 2.8 P, exposure energy Ed applied to the isolated-dot pattern and exposure energy Es applied to the isolated-line pattern are defined as Ed/Es greater than 3.0.
In this way, when exposure spot diameter D exceeds the value which is 2.8 times as large as dot pitch P, exposure energy Ed applied to the isolated-dot pattern is set to more than 3 times exposure energy Es applied to the isolated-line pattern. Then, according to the ratio of the exposure spot diameter relative to the dot pitch, optimum exposure energies are applied respectively to the isolated-dot pattern and the isolated-line pattern. Accordingly, without use of a lens processed and positioned precisely or a photoreceptor having its photosensitive layer decreased in its thickness for forming an image of a high resolution, a high-quality image can be reproduced with respect to both of the isolated-line and isolated-dot patterns.
According to the invention, when exposure spot diameter D and dot pitch P have a relation 1.9 P less than Dxe2x89xa62.8 P, exposure energy Ed applied to the isolated-dot pattern and exposure energy Es applied to the isolated-line pattern are defined as 1.5 less than Ed/Esxe2x89xa63.0.
In this way, when exposure spot diameter D is greater than the value which is 1.9 times dot pitch P and equal to or smaller than the value which is 2.8 times dot pitch P, exposure energy Ed applied to the isolated-dot pattern is set to a value more than 1.5 times exposure energy Es applied to the isolated-line pattern and equal to or less than 3 times exposure energy Es. Depending on the ratio of the exposure spot diameter relative to the dot pitch, optimum exposure energies are applied respectively to the isolated-dot pattern and the isolated-line pattern. Accordingly, without use of a lens processed and positioned precisely or a photoreceptor having its photosensitive layer decreased in its thickness for forming an image of a high resolution, a high-quality image can be reproduced with respect to both of the isolated-line and isolated-dot patterns.
According to the invention, when exposure spot diameter D and dot pitch P have a relation 1.4 P less than Dxe2x89xa61.9 P, exposure energy Ed applied to the isolated-dot pattern and exposure energy Es applied to the isolated-line pattern are defined as 1.0 less than Ed/Esxe2x89xa61.5.
In this way, when exposure spot diameter D is greater than the value which is 1.4 times dot pitch P and equal to or smaller than the value which is 1.9 times dot pitch P, exposure energy Ed applied to the isolated-dot pattern is set to a value more than 1.0 times exposure energy Es applied to the isolated-line pattern and equal to or less than 1.5 times exposure energy Es. Depending on the ratio of the exposure spot diameter relative to the dot pitch, optimum exposure energies are applied respectively to the isolated-dot pattern and the isolated-line pattern. Accordingly, without use of a lens processed and positioned precisely or a photoreceptor having its photosensitive layer decreased in its thickness for forming an image of a high resolution, a high-quality image can be reproduced with respect to both of the isolated-line and isolated-dot patterns.
According to the invention, preferably the surface of the photoreceptor having at the surface a photosensitive layer of 15 to 25 xcexcm in thickness is scanned by the exposure spot. The thickness of the photosensitive layer at the surface of the photoreceptor can thus be defined to prevent a considerable shortening of the life of the photosensitive layer and thus extend the life of the photoreceptor.
According to a preferred embodiment of the invention, power control is conducted by changing, according to image patterns, pulse height of a drive pulse supplied to a light source for each dot. By this power control, a drive pulse with its height controlled according to an image pattern to be formed is supplied to the light source. Then, the light source is driven by optimum drive energies respectively for the isolated-dot pattern and the isolated-line pattern. Consequently, respective exposure energies optimum for the isolated-dot and isolated-line patterns are applied.
According to a preferred embodiment of the invention, the width of a drive pulse supplied to the light source for each dot is controlled according to image patterns. By this pulse-width control, a drive pulse with its width controlled according to an image pattern to be formed is supplied to the light source. The light source is thus driven by optimum drive energies respectively for the isolated-dot pattern and the isolated-line pattern. Consequently, respective exposure energies optimum for the isolated-dot and isolated-line patterns are applied.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.